mellemrummellemrum(* = familiar, ** = very familiar, *** = vulgar, R = plain rude, Lit. = Literally)

Good

Una gozada (*) This noun applies to anything that's a very pleasant experience.
Es una gozada verle jugar (*) It's really great to see him play.

Guay (*) Cool. This is a catch-all adjective to describe anything that's cool.
¡Mira qué zapatos más guays! (*) Look! Such cool shoes

Guay del Paraguay (*) Lit. Cool from Paraguay. As cool as it gets - a silly rhyme that reinforces the idea of cool.
El móvil que se compró está guay del Paraguay (*) The mobile he bought is really, really cool.

Alucinar (*) Lit. To hallucinate or make hallucinate. To trip. It assumes the object you're praising has had such an impact it is comparable to an hallucinogenic drug.
Se compró un coche de alucina (*) Lit. He bought a car that makes you trip. He bought an incredible car.

Bad

¡Mierda! (**) Shit!
As in so many other languages, when something goes wrong there's an immediate cry announcing excrement.
¡Mierda, he vuelto a perder las llaves! (*) Shit, I've lost the keys again

...de mierda (**) Lit. ...of shit.
From that follows that whatever's gone wrong is made of excrement.
¿Dónde están las llaves de mierda? (*) Lit. Where are the keys of shit? Where are the damn keys?

Una mierda (**) Lit. A piece of shit. In Spanish you actually quantify it.
No me acuerdo porque tengo una memoria de mierda (*) I can't remember because my memory's rubbish.

Chungo/a (*) Dodgy.
Es una tía un poco chunga (*) She's a bit of a dodgy bird.

Hortero/a (*) Naff and corny.
You may also hear hortera, ending in -a, even when talking in the masculine. Also: una horterada, something as naff and camp as "knickers".
No te pongas esa chaqueta tan hortera (*) Don't wear that naff jacket.